Greg Madden from the Aviation Transport and Safety Bureau says the plane's wreckage is so badly damaged by fire that witness accounts will be needed to determine what went wrong.

"With the degree of damage sustained to the aircraft, it will make it a lot more difficult and witness reports will assume a [greater] importance than, say, if the plane was fitted with flight data recorders," he said.

Aviation safety investigators are hoping to have a preliminary report into the crash finalised within 30 days and the full investigation completed within 12 months.